1. Field of the Invention
The present invention relates to hydrotreating and aromatic saturation systems and method for efficient production of high quality distillates from high sulfur, high aromatic hydrocarbons at existing or new hydrocracking facilities.
2. Description of Related Art
Hydrotreating technology is a well-known prior art where hydrocarbon feed boiling in the range of from 150° C.-400° C. (302° F.-752° F.) is mixed with hydrogen at a temperature in the range of from 200° C.-500° C. (392° F.-932° F.) and a pressure in the range of from 34 barg-100 barg (493 psig-1450 psig) and the mixture is passed over heterogeneous fixed bed catalyst. The contaminants in the hydrocarbon feed, such as the sulfur, nitrogen, and oxygen compounds, are almost completely removed, and any olefins present are saturated, thereby producing products that are a mixture of essentially pure paraffins and naphthenes. Some of the aromatic content is also saturated. Acceptable product will meet the ultra low sulfur distillates specifications. The heterogeneous fixed bed catalyst contains at least one Group VIII metal, and at least one Group VIB metal. Generally, these metals are included on a support material such as alumina with or without silica or some other promoter.
The desired degree of hydrotreating takes place as the feed is processed over fixed beds of catalyst at elevated hydrogen pressure and temperature. The amount of catalyst required per volume of feed and the pressure level are set by the quality of the feed and desired products.
When there is a requirement of maximum aromatic saturation, the product from the distillate hydrotreating section is then further processed in an aromatic saturation reaction zone. The aromatic saturation of distillates is also a well known prior art, where the hydrocarbon feed is again mixed with hydrogen at temperatures in the range of from 200° C.-400° C. (392° F.-752° F.) and a pressure in the range of from 34 barg-100 barg (493 psig-1450 psig) and the mixture is passed over heterogeneous fixed bed catalyst. The heterogeneous fixed bed catalyst contains at least one Group VIII noble metal. Generally, these metals are included on a support material such as alumina with or without a cracking acidic component such as an amorphous silica alumina or a zeolite. The hydrocarbon feed is converted to higher-value low sulfur, low aromatic products, which are used as transportation fuel and meet the current Ultra Low sulfur distillate specifications.
The desired degree of aromatic saturation takes place as the essentially sulfur-free feed is processed over fixed beds of catalyst at elevated hydrogen pressure and temperature. The amount of catalyst required per volume of feed and the pressure level are set by the quality of the feed and the desired products.
Traditionally hydrotreating followed by aromatic saturation is carried out in multiple stages when the processes are combined into a single unit or carried out with two separate units. As the sulfur and aromatic content for a given distillation range in a hydrocarbon feed increases, the quantity of ammonia and hydrogen sulfide present in the hydrotreating zone effluent will also increase. The hydrogen sulfide will begin to inhibit aromatic saturation, and therefore in order to meet a high cetane number or smoke point for the particular distillate fraction, further processing is required. Catalytically this is achieved by increasing the hydrogenation function in the second stage-aromatic saturation zone. Since higher hydrogenation requires the use of noble metal catalyst which are poisoned by hydrogen sulfide, an intermediate fractionation section to strip out the hydrogen sulfide, ammonia and light ends is required. The stripped feed is then processed in the sweet (hydrogen sulfide free) second stage where aromatic saturation is carried out over a noble metal catalyst system followed by the fractionation section to strip out the hydrogen sulfide and light ends. This complicates the overall plant design and increases the amount of recycle gas required to achieve the desired targets.
Accordingly, a need exists in the art for improved hydrotreating and aromatic saturation processes operations, particularly for new systems capable of processing feedstocks with relatively high sulfur and aromatic content, or for existing systems which have been limited by catalyst activity and distillate selectivity.